This butter-based sauce is thick, creamy, and velvety — and isn’t as complicated to make as you might think.
In a saucepan, heat the wine, vinegar and shallots until the liquid boils, then lower the heat and continue simmering, about 10 minutes, until the liquid has reduced down to about 2 tablespoons. This should take about 10 minutes.
Once the mixture has reduced, remove the sauepan from the heat, and slowly, but rapidly whisk in the cubes of butter, one or two cubes at a time. As the butter melts and incorporates, add more butter and keep whisking. The finished sauce should be thick, smooth, and creamy.
Season with salt to taste. Straining the shallots is traditional, but optional.